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Discussion of Nitrate Loading Results <br /> Total Nitrate Impact <br /> Based on the methods and assumptions described, the result of the nitrate loading <br /> estimate indicates that the existing and proposed development on the Site has the <br /> potential to cause a buildup of nitrates in the aquifer over the MCL of 10 mg/L-N. <br /> There are several mitigating factors that should be considered, however, when <br /> evaluating the degree to which the Site is likely to impact ground water with nitrate: <br /> • The Hantzsche and Finnemore equation is a conservative method for <br /> determining potential impacts, and actual nitrate loading experienced on the Site <br /> over time may be less than predicted. The requirement by San Joaquin County <br /> Environmental Health Department that the calculation use rainfall and <br /> evapotranspiration data to determine ground-water recharge is particularly <br /> conservative, as it does not include consideration of recharge from irrigation <br /> water or surface water sources. <br /> • Depth to ground water beneath the Site is estimated to be approximately 50 to 63 <br /> feet. Given this depth to water, the vadose zone of the new septic system will be <br /> at least 25 feet, and significant nitrate attenuation would be anticipated over the <br /> course of this span. <br /> • Based on the map reviewed, nitrate levels in nearby wells have not exceeded the <br /> MCL despite the presence of numerous dwellings in the area; indeed, nitrate was <br /> not detected in half of the nearby wells. The nitrate level near the MCL at the on- <br /> site domestic well is likely due to the close proximity of the well to the existing <br /> septic system; it is also likely that this well is older and relatively shallow. <br /> • Based on the clay soil type present on the Site, it is likely that adsorption of <br /> nitrate ions to clay will occur as percolating effluent travels downward through the <br /> vadose zone. In their original paper, Hantzsche and Finnemore considered <br /> developments with relatively shallow groundwater; as a result, the soil factors <br /> included in the equation may not reflect the true nitrate reduction experienced as <br /> the effluent travels down through a larger vadose zone. <br /> Taking these factors into account, the actual nitrate loading experienced on the Site <br /> over time may be significantly less than predicted. <br /> LOGE 2325 Page 14 <br />